His refusal came as Rep. Devin Nunes of California, the House Intelligence Committee’s ranking Republican – who kicked off the hearing by calling for the testimony of the whistleblower whose complaint launched the impeachment investigation – asked witnesses to identify anyone outside the White House with whom they shared details of Trump’s phone call with Ukrainian President Volodymyr Zelensky.

Republicans used the exchange to raise questions about whether Vindman was the source for information that ended up in the whistleblower’s complaint, which alleged that Trump appeared to have abused his public office for personal political gain.

Vindman, a national security aide who listened with other officials to the July 25 call, told lawmakers during Tuesday’s House impeachment hearing that he believed Trump’s request for a foreign government to investigate a U.S. citizen was “improper.”

He said he provided a readout of the call to two individuals outside the White House as part of his responsibility to coordinate U.S government policy with multiple agencies. One was George Kent, a deputy assistant secretary at the State Department, who testified last week, and the other was an individual in the intelligence community, whom Vindman declined to name.

Vindman said both were “cleared U.S. government officials with the appropriate need to know.”

Growing frustrated when Vindman declined to provide a name of the second person, Nunes told Vindman he had a duty to either identify the people he spoke with or he would have to cite his Fifth Amendment right not to answer due to concern he would incriminate himself.

Citing the advice of his attorney, Vindman said he would not answer.

“Per the advice of my counsel, I’ve been been advised not to answer the specific questions about members of the intelligence community,” Vindman said.

House Intelligence Committee Chairman Adam Schiff, D-Calif., sought to halt the questioning, saying it was an effort to identify the whistleblower.

“We need to protect the whistle-blower,” Schiff told Nunes. “Please stop. I want to make sure that there is no effort to out the whistleblower through these proceeding. If the witness has a good faith belief that this may reveal the identify of the whistleblower, that is not the purpose that we’re here for.”

Nunes noted that Vindman he had testified in a closed-door deposition that he didn’t know the identity of the whistleblower.

“How is it possible for you to name the people and then out the whistleblower?” Nunes asked.

Vindman confirmed that he does not know the identity of the whistleblower and said he has been advised by his attorney not to answer questions about the members of the intelligence community. He said he first learned of the whistleblower’s complaint when the news media reported on it in September.

His attorney, Michael Volkov, said there was no need for Vindman to invoke his Fifth Amendment rights. He said that in refusing to answer, Vindman was following a rule set by Schiff regarding discussions that could implicate the whistleblower.

Vindman is the first person known to have sounded an alarm about Trump’s comments on the call. Immediately afterward, he told deputy White House Counsel John Eisenberg thatthe president had crossed a “disturbing” line in asking Zelensky to investigate former vice president Joe Biden, and said it was improper to ask a foreign government to investigate an American citizen. Eisenberg then instructed that the records of the call be placed on a secure server, telling Vindman he wanted to prevent leaks.

A few days later, Eisenberg confronted Vindman, saying he had just learned of an anonymous complaint from a CIA employee about the call, and wanted to know if Vindman had discussed the call with anyone else, according to a person familiar with his account.

Vindman said he had discussed it with two colleagues with whom he normally works on national security matters, and Eisenberg told him not to talk to anyone else about it in the future.